One reason that the paint is not adhering properly may be that it wasn't properly cleaned before applying the primer (as Laniegirl suggested), and another could be that the red underneath could possibly be an oil based paint in which case the latex primer would have a difficult time sticking to the oil. You should scrape/remove as much of the primer as you can, in any way possible, then start again. Use a primer, such as Kilz, tinted to match your gray color. You may need more than one coat of Kilz to cover the red - apply as many coats of this primer as necessary to achieve complete coverage.